Generalize DerivChristoffel in the CCZ4 system to allow tensor symmetry types used in the second-order CCZ4 system.

In the first-order CCZ4 system, $D_{kij}=\frac{1}{2}\partial_k \tilde{\gamma}_{ij}$ is an evolved field, so $\partial_l D_{kij}$ has no symmetry. However, in the second-order CCZ4 system, $\partial_l D_{kij}=\frac{1}{2}\partial_l \partial_k \tilde{\gamma}_{ij}$ is not evolved; it should have symmetry in k and l, since second partial derivatives commute for $C^2$ functions. This is also required to use second_partial_derivatives() in SecondPartialDerivatives.hpp. Hence, we generalize the deriv_conformal_christoffel_second_kind() within DerivChristoffel.hpp to allow for symmetric $\partial_l D_{kij}$ in the indices k and l.

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